1. Field of the invention
The present invention relates to a disk recording system, for recording a disk such as a compact disk. The present invention also relates to a method of controlling the rotation of a turntable relative to a recording head in a disk recording system.
2. Summary of the Prior Art
Many systems are known in which information is recorded on a disk-shaped medium and may subsequently be played back. Generally the information is arranged either in substantially circular rings or in a continuous spiral track on the disk. An example of the former arrangement is the magnetic floppy disk or hard disk, where the information is divided into sectors lying in concentric tracks. Examples of the latter arrangement include the conventional gramophone record carrying sound information in analog form in the spiral groove in its surface, the optically read videodisk carrying video information in analog form in a series of pits arranged spirally on the surface (or on an interfacial boundary) of the disk, and the compact optical disk carrying audio or other information in digital form in a series of spirally-arranged pits. The gramophone record, the videodisk and the compact disk are all examples of media available to the consumer which cannot normally be recorded on by the consumer; recording takes place on a master disk which is subsequently replicated by various processes such that the disks bought by the consumer are close copies of the geometry and the information content of the master disk.
The process of recording information on any of these media usually shares in common the fact that the disk or master disk is rotated at a speed typically anywhere between 16⅔ r.p.m. (for some gramophone records) and 1,800 r.p.m. (or even higher for some videodisks) while the point of recording (which may be a magnetic head, a mechanical stylus, or a focused light beam) is traversed between the inside and outer edge of the disk at a slower rate. Normally it is a requirement of the recording system that the rotational motion of the disk may vary only slowly, if at all; generally this is easily ensured by the inertia of the disk itself, together with that of the mechanism which rotates it. The radial motion of the point of recording on the disk is, however, not so easily controlled. In the case of magnetic disk recording, it is usual that the recording head must move in discrete steps between the separate concentric tracks; by contrast, in the cases of gramophone records, videodisks, or compact disks, the recording head must move continuously relative to the disk in a generally radial direction in order to lay out the information in a spiral track, and it is characteristic of these cases that the smoothness of the radial motion is more important than the absolute accuracy of radial positioning. With a gramophone record, for example, any radial motion having significant energy in the audio frequency band, even if it represents only a small fraction of the average groove spacing, will appear as a corresponding lateral movement of the pickup when the final record copy is played, and this will be audible as a noise superimposed on the recorded audio signal. With videodisks and compact disks there is not only the possibility that any sudden radial motions of the recording head will cause the player to fail to follow the track on the final disk, but also the more serious likelihood that such motions will be dangerous simply because they will result in significant changes in spacing between successive turns of the spiral track. Since this spacing is typically only 1.6-1.7 xcexcm, and any reduction in spacing has the effect of increasing the crosstalk between tracks (resulting in interference in the picture from a videodisk, or an increased likelihood of bit errors with a compact disk) it is desirable to maintain a tolerance of at most xc2x10.1 xcexcm in the track spacing, and preferably a much closer tolerance than this.
To obtain the necessary radial tracking motion, it is usual to move the recording head along a straight line which passes through the axis of the disk, in other words radially. When recording gramophone record masters this is commonly achieved by mounting the recording head on a linear slide or rolling mount and moving it by means of a rotating leadscrew and nut. Satisfactory performance is achieved by careful engineering; the stiffness of the leadscrew drive is great enough to overcome residual friction in the mounting in videodisk and compact disk mastering (recording) a similar technique may be used, in which the optics which produce the focused beam are moved over the rotating master disk. To avoid the disadvantage that part of the optics are thus movable while the remainder (owing to the size of the light source, normally a laser) have to be fixed, it is alternatively possible to move the entire turntable (which carries the master disk) together with its rotary bearing along a straight line, using a leadscrew, while the recording head remains fixed.
In long-playing optical videodisks, or optical compact disks used for audio or other data in digital form, a constant linear velocity mode or recording is normally used because it allows the maximum recording time consistent with operation at the optimum linear velocity (which determines the bandwidth of the signal which can be recorded) throughout the recording.
Constant linear velocity recording, however, adds complications to the system, because neither the speed of rotation of the disk, nor the speed of radial motion of the recording or playback head relative to the disk is constant. In a playback-only system this may be no great problem because both rotational and radial motions are normally controlled by servos governed by the information already laid down on the disk. Record/playback systems (for example read/write data recording systems) using pre-grooved disks are also simply implemented by servos of this type. However, in the case of a master recording system for videodisk or compact disk, where the master disk is initially devoid of groove structure, there is a problem in generating the required motions as it were from first principles.
If information is to be recorded in a spiral track of pitch P at a linear velocity v then, if the instantaneous radius is R and the rotational speed is xcfx89 (radians/sec.) at time t,                     ω        =                              v            R                    ⁢                      xe2x80x83                    ⁢          and                                    Equation        ⁢                  xe2x80x83                ⁢        1                                                      ⅆ            R                                ⅆ            t                          =                              P            ⁢                          xe2x80x83                        ⁢            ω                                2            ⁢                          xe2x80x83                        ⁢            π                                              Equation        ⁢                  xe2x80x83                ⁢        2            
from which                                           ⅆ            R                                ⅆ            t                          =                  Pv                      2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            R                                              Equation        ⁢                  xe2x80x83                ⁢        3            
EP-A-011495 discloses, in the context of defining or following a plurality of substantially circular and concentric information tracks, an arrangement in which the relationship of Equation 1 is obtained by generating a signal dependent on the radius R, and generating therefrom an alternating current signal of frequency inversely proportional to the radius R. The angular rotational speed xcfx89 of the turntable is then synchronised to that alternating current signal. The same alternating current signal is used to control the radial velocity,             ⅆ      R              ⅆ      t        ,
in accordance with Equation 2 using a lead screw drive. A similar method is described in EP-A-011493, in which an alternating current signal with a frequency inversely proportional to the radius R is generated, in this case by a digital division process.
It may be noted that the methods disclosed in EP-A-011495 and EP-A-011493 both require that an alternating current signal is generated first, to which the turntable rotation has to be synchronised.
As discussed above, it is possible for either the recording head or the turntable bearing to be made the moving element. However, whichever is the moving element, a lead screw drive system is not completely satisfactory in videodisk or compact disk mastering because of the great smoothness of motion required. A very precisely ground lead screw would be needed, and sticking or slipping of the lead screw mechanism could cause significant problems.
If a drive mechanism other than a lead screw is employed, generally it will be less stiff than a lead screw. The smoothness of the radial tracking motion is then strongly dependent on the attainment of very low friction in the bearing on which the recording head or (as the case may be) the turntable bearing unit moves.
In accordance with a first aspect of the present invention, the recording head and turntable are mounted so that the axis of the turntable is movable relative to the recording head, with the direction of that movement being defined by an air bearing.
The use of such an air bearing has the advantage of providing very low or essentially negligible friction in the desired direction of motion, combined with great stiffness to motion in other directions.
In order to control the relative motion of the recording head and the turntable bearing assembly, it is clear that in order to retain the full advantage of using a friction-free air bearing to support this motion (whether by moving the recording head or by moving the turntable bearing assembly) the driving mechanism should not of itself add friction. Friction-free means of causing relative motion are known, for example electric motors using a current-carrying coil of wire mounted in a magnetic field, or movable permanent magnets acted on by a variable magnetic field, or induction or hysteresis motors using movable electrically conducting or ferromagnetic elements acted on by a travelling alternating magnetic field. Generally such driving means have in common the property that in response to some control signal they will exert a controllable force on the movable element; however, when applied to a movable assembly mounted on an air bearing, they do not characteristically act in such a way as to define the position of the movable element, but only in such a way as to define its acceleration, since the mechanical behaviour of the moving element on its friction-free bearing is dominated by its inertia. This behaviour is in contrast to that of a leadscrew actuator, which is mechanically stiff and directly determines the position of the movable element. Additional suitable means are therefore necessary to control the position of the movable element.
It is well known in the art to control the relative motion of two elements by providing position sensing means to detect the motion, together with an amplifier responding to both the output of the position sensing means and an externally supplied control signal, the output from the amplifier providing the control input to the motor or other driving means in such a way that the resulting motion is constrained in accordance with the externally supplied control signal. In other words, this is a negative feedback servo-loop. Such a system characteristically has an upper frequency limit (or bandwidth) such that for motions having components predominantly below this frequency limit the motion is well controlled by the feedback loop whereas for motion having components predominantly at higher frequencies the feedback loop exerts little control.
In the arrangement described above, relating to the control of a massive assembly mounted on a friction-free bearing, the choice of bandwidth (which may be readily adjusted by changing, inter alia, the gain of a suitable amplifier) may be a difficult compromise. In the absence of feedback control a system consisting of a massive assembly mounted on a friction-free bearing is very vulnerable to external vibration, since in the presence of such vibration causing the normally xe2x80x9cfixedxe2x80x9d elements of the system to move, the tendency of what would normally be termed the xe2x80x9cmovingxe2x80x9d elements is to remain stationary owing to their inertia, resulting in a large relative motion between the two. If the bearing is a rotary one, the sensitivity to linear vibrations may be reduced by balancing the moving assembly. There remains, however, a sensitivity to vibrations which have a rotational component about the axis of the bearing. To suppress such relative motion the feedback loop must have a large bandwidth, covering all the frequencies at which external vibrations may be present.
If this is done the external vibrations are attenuated as regards relative motion between the xe2x80x9cmovablexe2x80x9d and xe2x80x9cfixedxe2x80x9d elements, but at the same time any noiselike or other fluctuations inherent in the signal generated by the position sensing means increase in importance; for the action of the negative feedback loop is such as to attempt to hold the reading obtained from the position sensing means to a set value, determined by the external control signal, so that the inherent fluctuations in this reading therefore appear, with opposite sign, in the actual position of the movable element. Specifically, those inherent fluctuations which fall within the frequency bandwidth of the negative feedback loop become imposed on the actual position of the movable element. Increasing the bandwidth therefore attenuates external disturbances but increases the effect of fluctuations in the position sensor reading. In the context of videodisk or compact disk mastering, it may be difficult to find a bandwidth great enough to remove the external vibrations which does not unacceptably increase the uncertainty caused by these inherent fluctuations.
It should be noted that the above discussion holds equally, whether the output of the position sensing means is directly representative of the relative position of the movable and fixed parts (so that the feedback loop acts in such a way that the external control signal controls the relative position), or whether the said output is representative of their relative velocity (in which case the external control signal controls their relative velocity). Similar considerations govern the choice of bandwidth in both cases.
There is therefore a need to reduce the effect of external vibrations without relying on a negative feedback loop of the high bandwidth.
In accordance with the first aspect of the present invention, there is provided passive damping of the motion of the movable elements, for example by a fluid-filled dashpot. With such a dashpot, an outer element is secured to e.g. a fixed element (or frame) of the system and an inner element is secured to the movable assembly mounted on the friction-free bearing described above. Such a dashpot offers viscous resistance to the motion of the movable elements. By contrast to the effect of inertia alone, which is such as to stabilise the motion of these elements with reference to an external (inertial) frame, the effect of such viscous damping is to tend to stabilise the motion of the movable elements with reference to the frame of the machine itself, and thus to attenuate rather than to accentuate any vibrations in the frame as far as relative motion between the xe2x80x9cmovablexe2x80x9d and xe2x80x9cfixedxe2x80x9d elements is concerned. Moreover, this attenuation occurs by purely passive means, and does not add noise or fluctuations as the negative feedback servo loop described above would have done.
It is necessary, by way of explanation to make a clear distinction between the effects of friction and the effects of viscous drag. Friction occurs when two solid parts are in contact, and is characterised in that in order to cause a sliding motion between the parts it is necessary to apply a force which exceeds a certain threshold, no matter how slow the desired motion may be. Viscous drag, as produced by a dashpot, by contrast sets up a force in resistance to relative motion which decreases as the speed of relative motion decreases, so that the rate of motion may be readily controlled by varying the applied force, down to the smallest speeds. It is an object of the present invention to eliminate friction forces by the use of an air bearing, and to replace them by viscous forces by the use of a dashpot.
To control the motion of such a system a negative feedback servo loop, as described above, may be employed. However, since the dashpot provides the means of attenuating the effect of external vibrations, and also since the required speed of motion changes only gradually with the radius of the recorded information on the disk in the case of constant-linear-velocity (CLV) recording, and may not change at all in the case of constant-angular-velocity (CAV) recording, this servo loop may have a small bandwidth (i.e. a long response time) such that any fluctuations introduced into the motion as a result of inherent noise in the position sensing means are not important. A bandwidth corresponding to a response time of 5-10 seconds has been found by the applicants to be suitable.
In accordance with a development of the first aspect of the present invention, a rotary bearing is used to support the relative movement of the recording head and the axis of the master disk in an arc rather than a straight line. In a preferred embodiment the recording head remains stationary while the turntable bearing unit moves, and the arc of relative motion is such that there is a position where the recording head lies directly on the axis of the master disk. This enables information or visible markings to be recorded as close to the centre of the final record as may be desired. Also in the preferred embodiment the axes of both the turntable bearing and the second rotary bearing which supports it are vertical, so that there is no gravitational force tending to move the turntable bearing unit one way or the other.
It may be thought that it is functionally inferior to allow the recording head to move in an arc relative to the axis of the disk rather than in a straight line. In fact, in the case of gramophone record mastering, since it is almost universal to play gramophone records by a stylus mounted on a swinging arm, it may well be that, if the master disk is recorded by a stylus moving relatively in a curve of similar geometry to that of the playback stylus, then the tracing distortion (arising during playback from the variation in orientation between the stylus and the recorded groove) may actually be less than with a linear motion. In the case of optical recording of videodisks of the xe2x80x9cLaservisionxe2x80x9d type and compact disks using a focused light spot, the recording process is not strongly sensitive to the orientation of the recording head to the recorded grooves. Furthermore, at least in the case of videodisks and compact disks recorded at a nominally constant linear velocity (CLV), the effect of the arcwise motion on the linear recording velocity is negligible notwithstanding that the arcwise motion has an angular component relative to the disk, because the arcwise motion is very slow compared with the rotational motion of the master disk.
The first aspect of the present invention may thus provide a viscous dashpot to damp and control the radial element of relative motion of recording head and master disk in a disk recording system where the relative motion is required to be in the form of a smooth spiral.
Preferably, the master disk rotates relatively rapidly on a first bearing and the relatively slow radial element of the said relative motion is supported by a second rotary bearing so that the radial element of the relative motion is a circular arc.
Preferably, the force to produce the relative motion is provided by a direct-drive electric motor using the moving-coil, moving magnet, induction or hysteresis principle.
Alternatively, the force to produce the said relative motion is provided by a spring, whose other end may be moved controllably by a geared motor assembly.
The first aspect of the present invention may also include a bearing arrangement for supporting the relative motion of the recording head and the master disk in a disk recording system where the relative motion is required to be in the form of a smooth spiral, in which the master disk rotates relatively rapidly on a first bearing and the relatively slow radial element of the said relative motion of the recording head and the first bearing is supported by a second rotary bearing so that the relative motion is a circular arc.
Preferably the second rotary bearing is then an air bearing, the entire assembly of the master disk on its turntable and bearing is supported on a bracket mounted on the rotor of the second rotary bearing, and the recording head is fixed.
The first aspect of the present invention may also include means for sensing the relative motion of two parts whereby a movable set of conducting elements may move in a transverse direction between two sets of fixed conducting elements carrying alternating voltages of opposite phase and the capacitively induced voltage on the said movable elements forms the input to a phase-sensitive detector, the reference input of which is the said alternating voltage applied to one set of fixed conducting elements, so that the d.c. output of the said phase-sensitive detector is a voltage representative of the relative position of the fixed and moving elements.
Preferably, the relative motion to be sensed is a rotational one, and the fixed and moving elements have the shape of circular sectors.
Preferably, an additional voltage representative of the rate of relative motion of the fixed and moving elements is obtained electronically from the voltage representative of the relative position of the same elements.
Preferably, the force to produce the relative motion is controlled by a linear servo amplifier whose inputs are the voltage representative of the rate of relative motion obtained as above and a reference voltage representative of the desired rate of relative motion.
A second aspect of the present invention will now be discussed,
From Equation 3, if R=0 at time to, then Equation 4 below holds:                     R        =                              {                                          Pv                π                            ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      t                    o                                                  )                                      }                                1            2                                              Equation        ⁢                  xe2x80x83                ⁢        4            
From this, it follows that:                     ω        =                              {                                          P                                  π                  ⁢                                      xe2x80x83                                    ⁢                  v                                            ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      t                    o                                                  )                                      }                                -                          1              2                                                          Equation        ⁢                  xe2x80x83                ⁢        5            
Thus, both R and xcfx89 are non-linear functions of time.
It would be possible to generate these functions of time in a digital computer, and feed the functions to digital to analog converters to obtain voltages representative of the required values of R and xcfx89. It would then be possible to use those voltages to control servo systems governing the radial and rotational motions. However, such an arrangement has the disadvantage that the values of R and xcfx89 thus obtained necessarily change in a stepwise manner, the magnitude of those steps depending on the resolution of the digital to analogue converters.
It would also be possible to use a digital computer to generate a value of the rate of change of radius R, given by equation 6 below.                                           ⅆ            R                                ⅆ            t                          =                              (                          Pv                              4                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  (                                      t                    -                                          t                      o                                                        )                                                      )                                1            2                                              Equation        ⁢                  xe2x80x83                ⁢        6            
This value, together with the value of xcfx89 derived by Equation 5, could be used to derive values which could then be output via digital to analog converters and fed to servos controlling the radial and rotational velocities. A single digital to analogue converter could be used for both values, since they have the same time dependence. Such an arrangement would have the advantage that there are not the stepwise variations in R, but only in its rate of change       ⅆ    R        ⅆ    t  
and the effect on the recorded disk is much less severe. However, the provision of an appropriate computing system means that complexity is high.
The second aspect of the present invention therefore seeks to obtain the necessary relationships between R and xcfx89 by analog means. At its most general, the present invention proposes that signals are generated corresponding to R and xcfx89; the product of these two signals is then used to control the rotation of the turntable in dependence on the difference between that product and a reference value.
A signal VR corresponding to the radius R is generated, as is a signal representative of and proportional to the rotation velocity xcfx89. The former is preferably a voltage signal and the latter is preferably a frequency signal. Such a frequency signal may be produced by a slotted strobe disk attached to a shaft of the disk recording system, which may be sensed by suitable optical means. The signal VR and the frequency signal may then be combined in a multiplying discriminator circuit which generates a suitable signal proportional to the product Rxcfx89. This product may be a voltage signal, which may then be compared with a reference voltage Vo representative of the desired linear velocity. The difference in voltages may then be used to drive a servo amplifier, which in turn drives a motor which rotates a turntable of the disk recording system.
Thus, with the second aspect of the present invention, it is possible to form a servo loop which tends to hold the product Rxcfx89 constant and equal to the desired linear velocity. A separate discriminator circuit may also be used to derive from the frequency signal a voltage Vc representative of the quantity             P      ⁢              xe2x80x83            ⁢      ω              2      ⁢              xe2x80x83            ⁢      π        ,
where P is the desired track pitch. This voltage Vc may then be compared with a voltage Vs derived from the voltage VR, which represents the quantity             ⅆ      R              ⅆ      t        .
The difference between the voltages Vc and Vs may then drive a servo amplifier whose output causes radial motion. Thus, the action of this second servo loop is such as to tend to hold Vs=Vc and thus to hold the radial velocity       ⅆ    R        ⅆ    t  
equal to the desired value             P      ⁢              xe2x80x83            ⁢      ω              2      ⁢              xe2x80x83            ⁢      π        ,
Preferably the disk recording system is used for optical recording of videodisks or audio or data compact disks.